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Radiation Protection during design, operation and dismantling of target areas. Thomas Otto based on material from RP group. Proton Target Areas at CERN. Radiation Protection Items. Ionisation and radiolysis Chemically Aggressive atmosphere (NO x , O 3 ) - PowerPoint PPT Presentation
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Radiation Protection during design, operation and
dismantling of target areas
Thomas Ottobased on material from RP group
IEFC Workshop 2010 - Thomas Otto 2
Proton Target Areas at CERN
AD 26 GeV/c 0.24 kW
N-TOF 20 GeV/c 1.25 kW
ISOLDE 1.4 GeV 3 kW
North 400 GeV/c 10’s of kW
CNGS 400 GeV/c 440 kW
IEFC Workshop 2010 - Thomas Otto 3
Radiation Protection Items
Ionisation and radiolysis Chemically Aggressive atmosphere (NOx, O3)
Activated air (spallation, activation) Release to the environment
Activation of ground and potentially ground water Environmental concern (3H, 22Na)
Material activation High ambient dose rates from activated material Future radioactive waste
IEFC Workshop 2010 - Thomas Otto 4
Air Ionization and Activation
Energetic secondary particles emerge from target
Traverse air, ionize and activate
A ~ (pathlength in air)
Estimate activation by heuristic formulae or Monte-Carlo simulation
Mainly short-lived beta-plus emitters 11C, 13N, 15O, 41Ar
IEFC Workshop 2010 - Thomas Otto 5
Purpose of “Nuclear” Ventilation
Create dynamic depression in the most radioactive area – protect other areas from leaking of activation/ contamination
Direct airflow to release point Allow release at a suitable point (e.g. at height) Permit aerosol filtering at unique point Permit monitoring at unique release point
Plus the usual purposes of ventilation, such as cooling, removal of noxious agents …
IEFC Workshop 2010 - Thomas Otto 6
Transport and Release of Air Two simplifying
cases: Laminar flow of air
volume at speed v (e.g. in tunnel)
Complete mixing of air in target volume, average dwell time
v v
Under these assumptions, activation release rate (Bq/s) can be estimated from ventilation speed
IEFC Workshop 2010 - Thomas Otto 7
ISOLDE 20072007: 3-times higher activity concentration at stack for equal proton beam current
“solved” by intensity limitationThe simplifying assumptions do not always work as advertised.
Small changes in air flow may have a big effect.
It took man-months of work at ISOLDE to re-establish release figures similar to those achieved before 2007
IEFC Workshop 2010 - Thomas Otto 8
Release vs. current
Activity production functions as expected !
IEFC Workshop 2010 - Thomas Otto 9
n-TOF 2009
Integrated release higher than expected
lack of tightness in target area requires higher extraction rate
Spikes ??From degassing system of cooling water: 15O
Solved by additional delay / decay tank
IEFC Workshop 2010 - Thomas Otto 10
Due to current interest:No directed extraction - PS
PS volume ≈ 24 000 m3
Ventilation injects 40 000 m3/h 1.7 exchanges / hour p(tunnel) higher than p(environment)
Smoke tests revealed: Uncontrolled release One reason for higher-than usual ambient dose
rates in PS ring
IEFC Workshop 2010 - Thomas Otto 11
PS Smoke Tests 2005
IEFC Workshop 2010 - Thomas Otto 12
Environmental model
A set of conservative models for transport of radionuclides into and in
the environment (by air and water, deposition, resuspension, uptake by crop and animals)
Exposure of representative group of public (irradiation, uptake)
Estimated dose can be compared with appropriate limits
Effluent (release)
Environmental transfer
Exposure Pathways
Dose Estimation
IEFC Workshop 2010 - Thomas Otto 13
Dose from CERN releases (Meyrin site, 2008)
For comparison:Dose to reference groups at Swiss NPP: PWR: < 1 Sv /y BWR: 5 Sv /y
Release*
(GBq)
Dose
Sv)
Short-lived beta emitters
13240 3.05
Other
(60 Co)
0.0092 0.22
Stray radiation - 3.7
Total 7.0
•PS underestimated (non-representative sampling point)
•PSB, EAST, AD at present not monitored•N-TOF 2009:
+5000 GBq, +1.1 Sv
Airborne releases are at the limit of the optimisation threshold. (10 Sv / y)
Any increase of releases will have to be justified and optimised
IEFC Workshop 2010 - Thomas Otto 14
Reduction of air-borne releases
Add shielding in target area Reduce track length in air
Closed ventilation system Cool and dry air in closed circuit Only vent before access
Release from height Construct a stack of h > 80 m
IEFC Workshop 2010 - Thomas Otto 15
Shielding in target area
Shielding absorbs secondary radiation cascade emerging from target
Less activation of air
Activation of shielding
IEFC Workshop 2010 - Thomas Otto 16
CNGS as an example
The target and horn are well shieldedCrane available for remote dismantling of shielding
IEFC Workshop 2010 - Thomas Otto 17
Closed ventilation system Cool & dry
activated air “never” release only before access
Appears a good idea, but Dynamic depression
difficult to maintain Accumulation of 3H
in condensate
TNM418m3
TSG430m3
TCV430m3
IEFC Workshop 2010 - Thomas Otto 18
Tritium – where from ?
Tritium is copiously produced as a light fragment in spallation reactions in (humid) air and in structural materials.
It diffuses out of the walls in form of HTO
In a closed ventilation system, the moist air is condensated in the dehumidifiers and the Tritium concentrated
IEFC Workshop 2010 - Thomas Otto 19
Consequence of CNGS – 3H
N-TOF 2008: “open” ventilation The concentration of short-lived
beta emitters allowed this solution
Tritiated water Storage in tanks Elimination via nuclear site in
France Evaporation – the concentration
in air is no reason to worry
IEFC Workshop 2010 - Thomas Otto 20
Release from height (stack)
Diffusion (dilution) coefficient for radionuclide concentration as a function of stack height.
High stack affords orders of magnitude better dilution in the close zone (< 1000 m) !
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
100 1000 10000 100000
Distance (m)
Dil
uti
on
6-15
26-35
>80
IEFC Workshop 2010 - Thomas Otto 21
Ground activation and ground water contamination
Soil around accelerator is inevitably activated Activation may be fixed or unreachable for
any human purpose (e.g. LEP / LHC) Activation migrates with ground water and
runoff water Fermilab experiments (1980’s):
7Be, 51Cr, 22Na, 54Mn, 46Sc, 48V, 55Fe, 59Fe, 60Co, 45Ca and 3H are produced
22Na, 3H are soluble and easily transported
IEFC Workshop 2010 - Thomas Otto 22
Distribution of activation
0 100 200 300 400 5001E-10
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
10
3H
Act
ivity
(B
q c
m-3)
Depth in Soil from Accelerator wall (cm)
5 , 2, 1 GeV
500 MeV
200 MeV
100 MeV
0 100 200 300 400
0.4
0.5
0.6
0.7
0.8
0.9
1.0
100 MeV 200 MeV 500 MeV 5 GeV
Fra
ctio
n of
cum
ulat
ed a
ctiv
ity
Depth in Soil from Accelerator wall (cm)
Lateral to proton accelerator tunnel:Exp. activation profile, = 38 cm
99.9 % of all activation is produced in a layer less than 3 m thick.(“activation zone”)
Water from the activation zone is exceeding rad. waste levels for 22Na and must be dealt with appropriately (infiltrations!)
Power of uncontrolled losses 1 W/m
IEFC Workshop 2010 - Thomas Otto 23
Protective layers
Drinking water resources outside the “activation zone” can be protected with a “geomembrane”
(example SNS)
IEFC Workshop 2010 - Thomas Otto 24
Situation at CERN
Accelerators and target areas close to the surface (Linac, PSB,PS, East, AD, ISOLDE, n-TOF, North) are far from drinking water sourcing areas
The potentially activated run-off must be diluted, in some cases in little streams with limited capacity
Infiltration water in tunnels and target areas may be activated beyond free release guidelines
IEFC Workshop 2010 - Thomas Otto 25
Waste from Target areas
Most target areas produce one-off waste, e.g. the ISOLDE front-ends or the n-TOF target
Highly activated, high ambient dose rate
Find special agreements with national repositories for elimination and final storage
IEFC Workshop 2010 - Thomas Otto 26
Waste from ISOLDE
ISOLDE produces 30 waste target units per year
High dose rate and actinide (alpha-emitter) content makes treatment in hot cell mandatory
Temporary storage saturated
Elimination pathway is required now
IEFC Workshop 2010 - Thomas Otto 27
Summary
Target areas have radiation protection topics in common Activation, radioactive releases, waste production
Imagine CERN increases beam power on targets: Ventilation must be well-designed, air-flow fully understood Release close to ground reaches its limits
closed circuit, high stack, shielding: no “silver bullet” Ground activation becomes an issue for close-to-surface
accelerators Target areas produce highly activated waste
Suitable interim storage and elimination pathways must be defined